IEEE Power & Energy Magazine - March/April 2021 - 68

security-constrained optimal power flow (SCOPF) solved
with mathematical programming].
Customized approximations aim at reducing the size
and/or complexity of the optimization problem by incorporating operational experience and knowledge about the
system. Ideally, an industrial-grade DSS for integrated
maintenance scheduling and congestion management
would combine multiple approaches that have shown good
performance in the literature, such as contingency filtering, network compression, and adaptive linearization. The
first two are typically implemented simultaneously, where
contingency filtering selects the contingencies that are
expected to be binding at the optimal solution, and network compression reduces the size of each postcontingency
network. Adaptive linearization could be employed to simplify the power flow equations of selected lines in formulations that use an ac network model. For instance, lines
with frequent N−1 violations or voltage violations at the
terminal buses are modeled with ac power flow equations,
whereas a dc model could be used for lines that hardly ever
experience violations.
Needless to say, the approach presented here can be
extended in various ways to account for additional issues
identified from the analysis of historical grid events. For
example, constraints on the degree of network connectivity
could be modeled in the " SCOPF for Congestion Management " module of Figure 11 to avoid creating weak grid links
due to remedial actions. Also, customized measures, such
as the minimum production product discussed earlier in the
" Historical Grid Events in the Swiss System " section, can
be obtained as the outcome of such an optimization-based
approach by introducing the appropriate modeling variables.
Finally, this approach could provide the basis for a more compressive (possibly probabilistic) medium-term energy security planning process that integrates outage planning, congestion management, and adequacy planning with consideration
of storage levels of hydro dams and market conditions.

Summary
In this article, we have addressed the challenging task of
maintaining power system security from the perspective
of the Swiss power system. We started with a review of the
distinctive characteristics of the Swiss electricity grid and
an explanation of today's practices for maintenance scheduling, operational planning, and real-time system operation.
Despite the success of these practices, we argue that enhancements or even a new operational paradigm are needed in
the future to cope with the constantly changing boundary
conditions of the power grid. Based on a few recent grid
events and historical trends in power system operation, we
identified aspects of particular importance. These include
representation of subtransmission and distribution grids in
planning and operation, better coordination of maintenance
scheduling across different power system assets, preventive
measures against voltage violations, and consideration of the
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effect of topological actions to the level of network connectivity. In response, we presented two key aspects of a new
operational paradigm that would help enhance system security, namely data analytics and DSSs.
Specifically, we outlined an integrated approach for the
traditionally time-separated activities of outage planning and
congestion management. The approach enables a combined
consideration of the maintenance needs of transmission, distribution, and generation assets in the planning phase while
increasing the margin for preventive interventions against
contingencies closer to real-time operation. To realize the
envisioned approach, methodological innovations in the field
of SCOPF are needed, including hybrid approaches combining ideas from mathematical and heuristic optimization.

Acknowledgments
The authors would like to thank the following Swissgrid colleagues who provided valuable input for the preparation of
this article: Christophe Dunand, Stéphane Gerbex, Adam
Kai, Rosmarie Joss, Thomas Reinthaler, Bastian Schwark,
Andrin Siegenthaler, Michael Wasmer, Christian Welti, and
Franziska Zuber.

For Further Reading
" System operation guideline (SOGL), " ENTSO-E, Brussels,
Belgium, 2017. [Online]. Available: https://www.entsoe.eu/
network_codes/sys-ops/
" Analysis of the grid security violation on 20 May 2019
available, " Swissgrid, 2019. [Online]. Available: https://www
.swissgrid.ch/en/home/newsroom/newsfeed/20190923-01
.html
" Planning for winter: Winter situation 2015-16, " Swissgrid, 2016. [Online]. Available: https://www.swissgrid.ch/
en/home/operation/regulation/winter-planning.html
P. C. López, R. Sadikovic, H. Pinto, and F. Magnago,
" Swiss TSO experience with an AC security-constrained
optimal power flow application for real-time security management, " in Proc. IEEE Eindhoven PowerTech, 2015, pp.
1-6. doi: 10.1109/PTC.2015.7232442.
A. Froger et al., " Maintenance scheduling in the electricity industry: A literature review, " Eur. J. Oper. Res., vol. 251,
no. 3, pp. 695-706, 2016. doi: 10.1016/j.ejor.2015.08.045.
F. Capitanescu, " Critical review of recent advances and
further developments needed in AC optimal power flow, "
Electric Power Syst. Res., vol. 136, pp. 57-68, July 2016.
doi: 10.1016/j.epsr.2016.02.008.

Biographies
Evangelos Vrettos is with Swissgrid Ltd., Aarau, 5001,
Switzerland.
Marc Hohmann is with Swissgrid Ltd., Aarau, 5001,
Switzerland.
Marek Zima is with Swissgrid Ltd., Aarau, 5001,
Switzerland.
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march/april 2021

https://www.entsoe.eu/network_codes/sys-ops/ https://www.entsoe.eu/network_codes/sys-ops/ http://www.swissgrid.ch/en/home/newsroom/newsfeed/20190923-01.html http://www.swissgrid.ch/en/home/newsroom/newsfeed/20190923-01.html http://www.swissgrid.ch/en/home/newsroom/newsfeed/20190923-01.html https://www.swissgrid.ch/en/home/operation/regulation7winter-planning_html https://www.swissgrid.ch/en/home/operation/regulation7winter-planning_html

IEEE Power & Energy Magazine - March/April 2021

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